How to divide a Pneumatic Conveying Stream

Pneumatic Conveying Lines

How to divide a Pneumatic Conveying Stream

Splitter for continuous distribution of bulk solid material during pneumatic conveyance - dimensioning, calculation, operating behaviour. Worldwide, a great number of so-called splitters are in operation as elements of pneumatic conveying lines, in order to continuously distribute the conveyed bulk material to several receiving stations. These splitters have taken an important position within the pneumatic conveying systems and are applied in different designs. But there is not much literature available dealing with the phenomenon “splitter”, and providing assistance regarding application possibilities, dimensioning, selection and experiences during operation. The available literature mostly deals with answers to specific questions, like for example pressure losses in various ramifications, etc.
(ed. wgeisler - 31/5/2017)
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1.5  Calculation of the Splitter

The main calculations for the splitter are, apart from the area division in functional dependence to the number of outlets, the sinking velocity, as well as the estimation of the additional pressure loss.

1.6  Calculation of the Sedimentation Velocity of Individual Particles

The sedimentation velocity of the individual particles w is calculated from the equilibrium of the weight force FG (perpendicular down) and the circulation force of the surge flow FW (perpendicular), neglecting the lift forces as follows:

where:

AS = projected area of the particle   [m2]
g = gravitational acceleration      [m/s2]
dS = cross section of particle               [m]
ρS = particle density                      [kg/m3]
cW = drag coefficient of particle           [–]

It should be noted that this equation only applies to individual particles; the sedimentation velocity of particle collectives is different, depending on its composition.

Usual sedimentation velocities of fine particles are in the range of 0,5 to 2,5 m/s. These values apply to an individual particle. Practical values can be determined through experimental determination of the sedimentation velocity, e.g. in the Gonell-classifier.

1.7  Calculation of the Pressure Loss in the Splitter

The pressure loss in the described splitter is mainly due to the re-acceleration of the solid particle/gas mixture. It is fairly accurately described in the following equation:

 

where:

Δpv = Pressure loss in splitter             [Pa]
K = Empiric factor  (value ≈ 1,2)         [–]
ρL = Gas density                             [kg/m3]
v  = Gas velocity                                [m/s]
μ  = Load                        [kg solid / kg gas]
c   = Solid particle velocity                [m/s]

At usual velocities, system pressures, temperatures and loads, the additional pressure loss of such a splitter generally varies in the range of 8 up to approx. 50 mbar.
 

2   Operation of the Plant

In order to achieve the most possible even division of the mass flow, some further points need to be taken into consideration while setting up the splitter (see Fig. 7 below):
● The splitter has to be set up perpendicularly.
● Before entering the splitter, the perpendicular incident flow length must be at least 15 (20 is more favourable) times dE (whereby dE is the inner pipe diameter at splitter inlet).


Fig. 7: Setup of a splitter with up-    
     and downstream lengths

An even division of the bulk mass flow rate can only be assured, if these minimum prerequisites are observed. It is recommended that a corresponding straight wake (10 to 15 times dA, dA = inner pipe diameter at splitter outlet) is also observed.

A disturbance-free operation of the conveying is of particular interest. The aim is to achieve accuracies in the range of ± 5 to 10%. The splitter is one part of a general plant, and in setting it up, fluidic aspects have to be observed, in addition to geometrical sizes.

It is important that the specific pressure loss occurs at the end of the conveying line, so that small differences in the lengths of feed pipes after the splitter can be neglected compared to this pressure loss. This can easiest be done by attaching a nozzle with a defined pressure loss to the end of the conveying pipe. A respective example is depicted in the diagram below (Fig. 8). Further explanations are provided in the following chapter.


Fig. 8: Typical pressure curve in a conveying system with a splitter                                                                                                             

 

The splitter lengths are dependent upon material characteristics, an additional air injection and the number of exits to be supplied. Usually, these lengths are in the range of 5 to 10 times the diameter of the splitter.

 

3   Homogenization of the Throughput

Fluid mechanic correlation at the splitter can be well described through a simplified electric circuit. The following is valid as an analogue:

Voltage U       ~ Pressure gradient
Current I         ~ Material throughput
Resistance R   ~ Resistance behaviour of pipe

The aim is always an evenly distributed throughput of material. Nevertheless, it often happens that due to local circumstances, the conduit after the splitter cannot necessarily be implemented absolutely even. This leads to the fact that both the resistances R1 to R4 and the throughput turn out differently, as they are directly linked with the pipe resistance (Fig. 9, below).

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